This invention is in the technical field of inspecting a curved surface of an object such as a soldered part on a printed circuit board.
Japanese Patent Publication Tokko 6-1173 describes a device for automatically inspecting a soldered part on a printed circuit board by a method of image processing by making use of the mirror reflection from the solder part. As shown in FIG. 9, this device serves to form an image of a target object to be inspected by means of three light sources 8, 9 and 10 emitting respectively red (R), green (G) and blue (B) light and an image pick-up device 3. Each of the light sources 8, 9 and 10 is disposed obliquely above the solder 2 to be inspected. With a device thus structured, each of the colored light beams from the sources 8, 9 and 10 makes incidence on the solder 2 at a different position corresponding to the angular orientation of its source. If the solder surface is sloped so as to be able to lead the mirror-reflected beam therefrom to the image pick-up device 3, a two-dimensional image is formed as shown in FIG. 10 with the colors R, G and B separated, corresponding to the positions of incidence of the beams.
In the example shown in FIG. 9, the three light sources 8, 9 and 10 are arranged such that the angles of their orientation measured from the horizontal plane increase in the order of R, G and B, and these angles are determined according to the portions of the curved surface to be separately detected. Thus, if a dominant color is extracted at each point on the image obtained by the irradiation from the sources 8, 9 and 10, each color component can be clearly distinguishable as shown in FIG. 11, corresponding to a flat surface portion where the surface is nearly flat and the sloping is the smallest, a steeply sloped surface portion where the surface is the steepest, and a gently sloped surface portion in between.
Since a two-dimensional image can thus be obtained with each of the colors R, G and B separated according to the sloping angle of the solder surface, it is possible to determined the quality of the surface condition by preliminarily registering the pattern of each color on an image of a desired solder surface shape and comparing the color patterns on an image obtained from a target object of inspection.
Recently, however, lead-free solder is coming to be used more frequently by the makers of circuit boards in view of environmental problems. Since lead-free solder is a non-eutectic alloy with constituent metals having very different solidification temperatures, small protrusions and indentations are likely to be developed on its surface. As a result, lead-free solder has a higher diffusive reflectivity than conventional eutectic solder with lead and tin as principal constituents. Moreover, the brightness of each light source is usually adjusted, for the ease of visual inspection by the operator, such that a white color will result if the diffused light from each source is mixed together. Thus, if an image is formed of an object with a high diffusive reflectivity such as lead-free solder, the image as a whole becomes white as each color component is mixed together or the boundaries between color patterns become unclear and obscure. When the aforementioned adjustment is made, furthermore, a white-colored diffusive reflecting plate is disposed with its planar surface horizontally oriented and an image of its diffusive reflecting surface is obtained such that the operator can make adjustments while referencing the color of the diffusive reflecting surface on the image. Thus, the red color on the flat solder surface portion close to the disposed diffusive reflecting plate becomes particularly difficult to recognize visually.
At the time of teaching operation with the inspection device, an image of a model soldered part is displayed and the operator is required to set a threshold value for binary conversion for extracting each color pattern and a judgment reference value to be used for judging whether or not an extracted color pattern is acceptable or not. If a whitish image is generated because the color components are mixed together or if the boundaries between the color patterns become unclear, however, it is not easy and may take a long time to visually determine an appropriate threshold value for binary conversion. This optical system may be used for a visual inspection device, but it takes a long time to evaluate the condition of each target area to be inspected if it becomes difficult to ascertain subtle color differences at boundary areas between different color patterns.
If the diffusive reflectivity of the lead-free solder to be inspected increases, furthermore, the difference in brightness gradation of the color components becomes smaller, making it difficult to maintain a high level of accuracy in the inspection.